The hierarchy of organ, tissue, cell in the human body

As another new semester begins, what better place to begin than the hierarchy of structure in the human body — organ systems, organs, tissues and cells. I hope this doesn’t sound old-fashioned, but it’s a strict hierarchy!

It sounds easy enough, but it’s easy to lose your bearings when you hear statements like “bone tissue has a lot of blood vessels”.  Sure, that’s true — and that’s why bone heals faster than, say, cartilage.  But this does not mean that blood vessels are a part of the bone tissue.  Blood vessels, in fact, are made of tissues like muscle, connective tissue, epithelium. So, if a blood vessel were a part of the bone tissue, then you’d have one tissue that’s part of another tissue — bad news for our hierarchy!

Osteoblast settles down and changes her last name

Many people these days are aware that their bones are in a constant state of flux, and that’s just one more reason to get some exercise — the mechanical stresses tell the cells in your bones to add more bone matrix (the nonliving material between the cells — mostly the hard mineral calcium phosphate, but also collagen and other ingredients).

There’s an important transition in the life of your bone cells. The early, “young and restless” cells that travel around and secrete bone matrix, sowing their wild oats, so to speak, and perhaps “having a blast”, are conveniently known as osteoblasts.   Gradually, most of the space gets filled in with hard matrix, and eventually the osteoblasts become trapped in their own secretions.  Once the cell is forced to settle down “on site” in this way, it is (again conveniently) known as an osteocyte.

The metaphor of “Mrs. Osteocyte” having “changed her last name” from “blast” to “cyte” was too perfect to resist, although I must point out that the metaphor relies on gender stereotypes that are rapidly becoming obsolete.

Anyway, Mrs. Osteocyte has plenty of work left to do and is not at all isolated, as it might seem. She maintains a firm grip on all her neighbors, sharing nutrients, wastes and chemical signals with them, directly through gap junctions. Indeed, osteocytes play an important role in detecting the mechanical forces that act on the bone, and secreting signals that direct the activity of osteoblasts. So, in effect the older generation still calls the shots!


Pancreas adds the special sauce

Last time we looked at a gland in the stomach where acid and an enzyme are secreted to begin the breakdown of protein.  I talked about the chyme that is produced by the mixing of stomach secretions with the food. As chyme leaves the stomach (symbolized by the hamburger — although in reality it is now mush), it enters the duodenum (shown here as a pink tube) — the first part of the small intestine, and by far the shortest  (the original term “duodenum digitorum” meant it was “twelve fingers’ widths” in length).  It’s a tiny part of the 10-foot long small intestine, but a vital part, because this is where several key ingredients are added.

The pancreas is far more important for digestion than the stomach because it secretes enzymes to digest all three food groups — fats, carbohydrates, and proteins.  It’s a somewhat fish-shaped gland, that sits behind the stomach.  These enzymes don’t function well in an acid environment, so the pancreas throws in a little natural “alka-seltzer” (bicarbonate) to neutralize the acid, and all this is poured together into the duodenum, through a duct.

The word pancreas means “all flesh” because there is little in the pancreas other than soft, secretory cells (nothing tough, no gristle).  I tried it once in New Orleans (it’s called “sweetbread” on the dinner table), and it wasn’t bad at all.

A gutsy first post

This view of a gastric gland marks the first post in this blog about biological cartoons.  The goal of the blog is to share some of the cartoons I’ve been using in my classes, and meet up with like-minded folks who share my view — that most lectures and nonfiction books are better off with a few cartoons. I will also broaden the discussion at times to look at the whole genre of educational cartooning, and share with you some of the best examples I’ve found.

But my training is in biology, so that will be my main focus.  The choice of the gastric gland is perfect, not just because I will be lecturing on it tomorrow (really!), but it also represents the anxiety (and a little extra stomach acid) I have in launching this project. I have never done a blog before; it sounded like too much time commitment.  But as the cartooning has become central among my interests, I realized it was time to move beyond the powerpoint slides and reach out — I’m hoping at the very least, there are a handful of teachers out there who might find a place for these cartoons in their classroom.

Since space is not an issue here, what I will do is show the cartoons unlabeled, and provide explanatory text, with the main terms in bold.  The drawing shows the main cells of the stomach epithelium (orange) secreting mucus, which protects the epithelium from its own secretions. Nevertheless, there’s rapid turnover of cells (replaced every few days), and a couple hapless oldsters are shown plunging into the roiling cauldron of stomach acid and food, known as chyme. This fortunate term (which, happily, comes from the same Indo-European root as “humor”!) gives rise to a plethora of puns, one of which is posted on the “wall” next to the “police chief” cells representing chief cells.  These cells secrete pepsinogen, an enzyme precursor, which when activated (by acid) digests proteins in the stomach. So the police chief is truly “taking a bite out of chyme”, which is a reference to the “take a bite out of crime” ad campaign from the 80’s.

Stomach acid is secreted by parietal cells (blue), which are the most realistic drawings in the figure — their “teeth” are the many microvilli (convolutions of the membrane) that add surface area, maximizing the ability to secrete acid. At the bottom of the gland is an enteroendocrine cell (purple), a general term for the many epithelial cells of your gut that secrete hormones.  The cell is “tasting” the food molecules in the gland, and sending out a hormone into the bloodstream to activate other glands of the stomach. Finally, the pink cells with pacifiers are (you guessed it) baby cells, or stem cells, which have a sort of “immortality”, continuing to divide throughout your life, while their progeny migrate up and down to become the replacement cells in the epithelium.